I have had the 2008 MKX for about 3 months. Most of the time the navigation system works fine. However, at times the sound will just go out. The stereo will lower, but the voice that gives the directions from the nav. sys. cannot be heard. The signal that a turn is to be made, also cannot be heard. It will eventually start working again--in a day or so. The dealer cannot find the problem. Has this occurred for anyone else?